Implantable drug infusion devices usually are implanted for several years, during which time there is no opportunity to service or repair these devices. Accordingly, both the implanted device and the drug to be infused must remain operative and be stable for a long period.
The current trend is toward extending significantly the intended life of implanted drug infusion devices. Further, new drugs having different physical and chemical properties are continually being developed, the use of which may not be sufficiently compatible or properly interactive with current devices.
Conventional implantable fluid delivery systems for drug infusion typically include a reservoir to store the fluid, and a separate pump or other flow control device to deliver the fluid. This type of implantable drug infusion system is shown generally in FIG. 1. Reservoirs in conventional implanted drug infusion devices undergo little motion, so that the materials which form the reservoir can be selected primarily for compatibility with the drug that is to be delivered. Pumps, on the other hand, typically have many moving parts. The materials from which the pumps are made thus must be selected for specific mechanical properties, such as flexibility, durability, and strength. The materials used may not exhibit all desirable mechanical properties, or may not be compatible with some drugs to be infused. An example is a silicone tube in a peristaltic pump. Silicone is very flexible and durable, but is permeable to many fluids and is not as strong as other elastomers.
With respect to another incongruity of design objectives, because implantable infusion devices preferably are small in size, the devices typically have only limited internal space, so that any materials used in the manufacture of the device must be of the type that can be used to manufacture very small parts. One example of such a material is silicon formed by micromachining. Silicon, however, is difficult to join to other materials.
Along the same lines, although it is desirable to have implantable fluid delivery systems that are small in size, the reduced size of certain conventional devices can be problematic in some applications. For example, pumps or valves with very small passages are not able to effectively deliver certain fluids. Some drugs, such as insulin, often are damaged by shear forces when flowing through such small passages.
Finally, another design incongruity arises with conventional drug infusion devices in which the drug is routed from the reservoir to the pump, and then to the device outlet where it is delivered to a desired location within the body. A certain amount of the drug thus is contained in the pump and fluid passages. This fluid, held in what commonly is referred to as the "dead space," must be displaced whenever the drug in the reservoir is changed. Delivery systems with relatively more dead space require more unused, unwanted drug to be infused before the drug can be changed.